1. Field of the Invention
The present invention is directed to a method for determining the mass flow of gases on the basis of optical absorption.
2. Description of the Related Art
Fluid and solid initial materials or substances that have a low vapor pressure and a low vapor pressure in comparison to previously standard gases such as, for example, SiH.sub.4, B.sub.2 H.sub.6 are being increasingly employed in CVD coating processes (chemical vapor deposition) in many areas of technology, for example in microelectronics or in anti-wear protection. This development results, first, for safety reasons (if any contamination occurs at all given potential accidents, this should only be a local contamination) and, second, from method-conditioned simplifications. In the latter instance, layers should also be capable of being produced from compounds by simple, thermal decomposition of an initial substance. A typical example is the employment of TEOS (tetra ethyl ortho silicate) for the deposition of SiO.sub.2 in semiconductor technology. This fluid has a vapor pressure of a few hundred Pa at room temperature. Metallo-organic compounds for metal deposition are usually present in solid form at room temperature. The vapor pressure often lies below 100 Pa. Their reliable and time-constant dosing is an indispensable prerequisite for the reproducible deposition of layers.
Commercially available apparatus that are referred to as "mass flow controller" or "mass flow meter" are usually employed now for designational dosing of gaseous substances. These devices measure the mass flow of the gaseous substance via the thermal conductivity.
Such apparatus work on the principle that a pressure drop is produced in a line for the gaseous substance to be investigated, for example, by diaphragms or laminar elements. Dependent on the pressure difference, a part of the gaseous substance can flow via a bypass capillary arranged parallel thereto. Heating wires that typically heat the gaseous substance to 70.degree. through 100.degree. C. are attached to this capillary. Thermal elements that measure the temperature distribution are also thereby provided.
When no gas flows through the line, then a pressure drop does not arise in the system and, thus, a flow does not arise in the capillary. A uniform, symmetrical temperature distribution therefore derives. The deviation from this distribution that derives given a flow can form the basis for the mass flow as a quantity. Apparatus of the abovementioned type as well as their function are described, for example, in a company brochure "V-MAX The ultimate vapor source" of Tylan GmbH in Eching/Munich.
One problem given such apparatus is that an admission pressure of at least 10.sup.3 through 10.sup.4 Pa is required for the measuring principle in order to produce the pressure drop in the measuring system. This minimally required admission pressure can only be produced at higher temperatures of, for example, more than 100.degree. C. given many substances or materials, being produced via their vapor pressure. The following disadvantages derive due to the necessity of heating the materials or, substances:
In part, the materials are not adequately thermally stable and decompose before the necessary vapor pressure is reached.
Commercially available "mass flow controllers" are normally limited to an operating temperature of 70.degree. C. This temperature can be somewhat higher only in special designs.
Other disadvantages are that the measuring principle loses precision, the useful signal becomes extremely low and the apparatus can at most be operated only at a fixed temperature.
A further limitation in the use of mass flow controllers is that they must be calibrated to a specific material or, to a specific substance since each substance has a different thermal conductivity. This makes universal employment impossible or limits this decisively. Over and above this, the thermal conductivity data are often not known or are only very imprecisely known.
A further possibility of dosing materials having low vapor pressure uses an inert carrier gas. The carrier gas thereby flows through a container and a saturation of the carrier gas according to the partial pressure ratio of the materials occurs in the ideal case.
As a rule, however, an absolute control of the mass flow is thereby not possible, since the actual degree of saturation can fluctuate greatly and, given employment of mass flow controllers, the carrier gas also contributes to the thermal conductivity.
In general, the following measures are currently practiced for dosing materials.
No control of the material flow ensues, this in fact being the simplest solution in terms of technology and equipment but not offering any possibilities of monitoring whatsoever.
The employment of mass flow controllers and mass flow meters represents a simple monitoring possibility that can be employed for low temperatures from 40.degree. through 50.degree. C. When work must thereby be carried out with a carrier gas and at higher temperatures, then this is also unusable for the afore-mentioned reasons.
Given a pressure regulation, the pressure in the evaporator system through which the carrier gas flows is held constant with pressure sensors and control valves. One thereby proceeds on the basis of an unaltering saturation of the carrier gas with the substance to be doped according to the vapor pressure ratio. One disadvantage, however, is that, first, the vapor pressure curve of the material must be exactly known--this usually not being the case and, second, one must proceed on the basis of ideal conditions--among other things, a long dwell time of the carrier gas in the evaporator.
In summary, all previously known possibilities are not satisfactory, particularly in the temperature range above 50.degree. C., and that a reliable, absolute statement about the actual mass transport of a material during the course of a process, for instance in a CVD process, is not possible.